But whether children without vitamin deficiency would benefit remains unclear, study finds

The study included nearly 250 schoolchildren in Mongolia with low blood levels of vitamin D during winter. Taking a daily vitamin D supplement reduced their risk of respiratory infection by half, the researchers said.

People in Mongolia are known to be at high risk for vitamin D deficiency, especially during winter. The low levels of vitamin D among the Mongolian children in the study are relatively common in some groups of Americans, particularly black children who live in northern states, the researchers said.

The children in the study were given 300 international units (IU) of vitamin D a day, which was higher than the recommended dose when the study began. The U.S. National Institute of Medicine now recommends a daily dose of 400 IU for children, while other groups recommend daily doses of up to 1,000 IU for children at risk of vitamin D deficiency.

The study provides "strong evidence that the association between low vitamin D and respiratory infections is causal and that treating low vitamin D levels in children with an inexpensive and safe supplement will prevent some respiratory infections," study corresponding author Dr. Carlos Camargo, of Massachusetts General Hospital, said in a hospital news release.

"The large benefit was undoubtedly related to the low baseline vitamin D levels of these children, so I would not expect the supplement to provide similar benefit in children who start with healthy levels of vitamin D," added Camargo, who also is a professor of medicine at Harvard Medical School in Boston. "The key question for future research is at what initial vitamin D level would children no longer receive benefit from winter supplementation?"

The study was published online Aug. 20 in advance of print publication in the journal Pediatrics.

Vitamin D is best known for its role in bone health, but many recent studies have shown that it has other important roles, including in immune function.

The body naturally produces vitamin D in response to sunlight. Maintaining adequate levels of the vitamin in winter can be particularly challenging for people who live in certain areas -- such as the northern United States and Canada -- where there are major seasonal variations in daily sunlight.